Vertical Accretion Research Articles

Effects of Stem Density and Reynolds Number on Fine Sediment Interception by Emergent Vegetation

Suspended sediment collected by vegetation in marshes and wetlands contributes to vertical accretion, which can buffer against rising sea levels. Effective capture efficiency (ECE), a parameter quantifying the fraction of incoming suspended particles directly captured by underwater vegetation surfaces, plays a key role in determining the significance of direct interception in morphodynamic models. The ways in which physical characteristics of collectors and transitionally turbulent flows affect ECE are not yet thoroughly understood. We conducted a set of 12 experiments at three flow velocities and three stem densities (plus equivalent zero-collector control experiments), plus four experiments where biofilm was allowed to accumulate. We determined that ECE decreases with increasing collector Reynolds number (study range: 66 to 200; p < 0.05 for two of three treatments) and increasing collector density (solid volume fraction: 0.22% to 1.17%; p < 0.05 for two of three treatments). Adding biofilm increased ECE in all cases, by a multiplicative factor ranging from 1.53 to 7.15 at different collector densities and biofilm growth durations. In some cases, the impact of biofilm on ECE far outweighed that of collector Reynolds number and density. By combining our data with those of one similar study, we present a preliminary model quantitatively assessing the effect of collector density on ECE.

Sea-level rise enhances carbon accumulation in United States tidal wetlands

Sea-level rise enhances carbon accumulation in United States tidal wetlands

Episodically volatile high energy non-cohesive river-floodplain systems: New information from the Ping River, Thailand, and a global review

Volatile rivers that involve floodplain stripping and subsequent floodplain reconstruction by vertical accretion are poorly known worldwide. This paper aims to partially fill this knowledge gap by a review of existing information and the provision of the currently most detailed account of such a river, namely the Ping River of northern Thailand. Recognition of this river type depends upon stratigraphic and morphostratigraphic analysis and so does not come within the ambit of those who focus only on river form and modern flood hydrology, and also not within the ambit of ‘traditional’ palaeoflood hydrology. A morphostratigraphic analysis of the Ping River adds to the small but valuable corpus of existing studies and provides an indication of the level of detail that is required for an in-depth understanding. A global review shows that extreme rainfall is always involved in floodplain stripping, although the sensitivity to rainfall extremes of catchments is not understood, including the contributions of land use, topography and alluvial resistance to erosion. The Ping River has the same characteristics as other volatile rivers, with an average interarrival time for stripping events of about three centuries probably caused by different combinations of wet periods, tropical lows, typhoons, and atmospheric rivers. As the intensity of extreme rainfall increases as Earth warms, the frequency of floodplain stripping events may increase with implications for both the pace of change in some fluvial landscapes and flood mitigation strategies.

Accretion, retreat and transgression of coastal wetlands experiencing sea-level rise

Abstract. The vulnerability of coastal wetlands to future sea-level rise (SLR) has been extensively studied in recent years, and models of coastal wetland evolution have been developed to assess and quantify the expected impacts. Coastal wetlands respond to SLR by vertical accretion and landward migration. Wetlands accrete due to their capacity to trap sediments and to incorporate dead leaves, branches, stems and roots into the soil, and they migrate driven by the preferred inundation conditions in terms of salinity and oxygen availability. Accretion and migration strongly interact, and they both depend on water flow and sediment distribution within the wetland, so wetlands under the same external flow and sediment forcing but with different configurations will respond differently to SLR. Analyses of wetland response to SLR that do not incorporate realistic consideration of flow and sediment distribution, like the bathtub approach, are likely to result in poor estimates of wetland resilience. Here, we investigate how accretion and migration processes affect wetland response to SLR using a computational framework that includes all relevant hydrodynamic and sediment transport mechanisms that affect vegetation and landscape dynamics, and it is efficient enough computationally to allow the simulation of long time periods. Our framework incorporates two vegetation species, mangrove and saltmarsh, and accounts for the effects of natural and manmade features like inner channels, embankments and flow constrictions due to culverts. We apply our model to simplified domains that represent four different settings found in coastal wetlands, including a case of a tidal flat free from obstructions or drainage features and three other cases incorporating an inner channel, an embankment with a culvert, and a combination of inner channel, embankment and culvert. We use conditions typical of south-eastern Australia in terms of vegetation, tidal range and sediment load, but we also analyse situations with 3 times the sediment load to assess the potential of biophysical feedbacks to produce increased accretion rates. We find that all wetland settings are unable to cope with SLR and disappear by the end of the century, even for the case of increased sediment load. Wetlands with good drainage that improves tidal flushing are more resilient than wetlands with obstacles that result in tidal attenuation and can delay wetland submergence by 20 years. Results from a bathtub model reveal systematic overprediction of wetland resilience to SLR: by the end of the century, half of the wetland survives with a typical sediment load, while the entire wetland survives with increased sediment load.

Types of the Cambrian platform margin mound-shoal complexes and their relationship with paleogeomorphology in Gucheng area, Tarim Basin, NW China

Based on the observation of outcrops and cores, thin section identification, restoration of paleogeomorphology by residual thickness method, fine description of seismic facies, etc., the coupling relationships between the development patterns of various types of Cambrian platform margin mound-shoal complexes and paleogeomorphology in the Gucheng area of Tarim Basin have been examined. The Cambrian platform margin mound-shoal complex is divided into mound base, mound core, mound front, mound back and mound flat microfacies, which are composed of dolomites of seven textures with facies indication. The different paleogeomorphology before the deposition of mound-shoal complex in each period was reconstructed, and three types of mound-shoal complex sedimentary models corresponding to the paleogeomorphologies of four stages were established: namely, the first stage of gentle slope symmetric accretion type, the second stage of steep slope asymmetric accretion type and the third and fourth stages of steep slope asymmetric progradation type. Their microfacies are respectively characterized by “mound base - mound back + (small) mound core + mound front - mound flat” symmetric vertical accretion structure, “mound base - (large) mound core + mound front - mound flat” asymmetric vertical accretion structure, “mound base - (small) mound core + mound front - mound flat” asymmetric lateral progradation structure. With most developed favorable reservoir facies belt, the steep slope asymmetric accretion type mound-shoal complex with the characteristics of “large mound and large shoal” is the exploration target for oil and gas reservoir.

Largest marsh in New England near a precipice

The Great Marsh in northern Massachusetts and southern New Hampshire is the largest salt marsh in New England and includes the marsh surrounding Plum Island Sound, one of the most studied coastal wetlands in the United States. A primary goal in marsh research today is predicting their resilience in a regime of sea-level rise, which has become increasingly based on modeling studies with sparse or site-specific field data input. Here, we present the results of a vertical accretion study using 210Pb and 137Cs geochronology developed from 50-cm long auger cores taken at 15 locations throughout the PIS marsh. Two significant outliers were omitted from the final analyses due to their unusual depositional settings. The 137Cs spike produced the shortest-term record and lowest average accretion rate (2.5 mm/yr). Of the various 210Pb accretion models (Constant Flux-Constant Sedimentation: CF-CS, Constant Rate of Supply: CRS, Constant Initial Concentration: CIC), we chose the CF-CS model accretion rate for comparison to other environmental parameters and future marsh projections due to the robust R2 linear regression values and its wide use. The 13 stations exhibited an average accretion rate of 3.13 mm/yr, which is 0.6 mm/yr greater than the average rate reported elsewhere in New England. Individual accretion rates show no correlation with geography, elevation, bulk density, loss on ignition, distance to tidal channels, or peat thickness, indicating the complexity of the interacting factors that control marsh growth. Using moderate sea-level rise predictions, we show that large portions of the high marsh are likely to be converted to low marsh by 2050, and that the entire marsh high platform will become low marsh prior to 2070. This is the first time that wide spread field data are used to project a major transformation of the Plum Island Sound platform marsh that undoubtedly will lead to dramatic changes to marsh drainage, backbarrier hypsometry, tidal current dominance, and redistribution of sediment reservoirs, that will include the fronting barrier islands.

Livestock grazing reduces sediment deposition and accretion rates on a highly anthropogenically altered marsh island in the Wadden Sea

Coastal salt marshes and their provided ecosystem services are threatened by rising sea levels all over the world. In the Northern Wadden Sea region, a sea-level rise of 4 mm y−1 was recorded for recent years. Identifying and understanding factors that affect sediment deposition and determine vertical accretion of salt marshes is crucial for the management of these ecosystems. Even though major processes contributing to sedimentation and accretion have already been identified, the influence of reduced canopy heights due to livestock grazing is still debated. On a highly anthropogenically altered marsh island in the Wadden Sea, sediment deposition, accretion and suspended sediment concentration was analyzed on grazed and adjacent ungrazed plots both at the marsh edge and at the marsh interior. Due to a low seawall (a so-called ‘summer dike’), flooding frequency on the island is reduced and flooding mainly takes place during storm surges. After five flooding events within a year, mean sediment deposition and accretion were found to be up to seven times higher on ungrazed plots compared to grazed plots, but only at the marsh edges. This result was not explained by the overmarsh suspended sediment concentration (SSC), which was found to be twice as high on grazed plots compared to ungrazed plots. It is concluded that grazing has a negative effect on sediment deposition and accretion on Wadden Sea marsh islands and areas with similar conditions (e.g. presence of a summer dike) by reducing the sediment trapping capacity of those marshes. Overall, vertical marsh accretion ranged from 0.11 ± 0.09 mm y−1 on a grazed plot at the marsh edge to 1.12 ± 0.71 mm y−1 on an ungrazed plot at the marsh edge. By increasing the discrepancy between accretion and sea-level rise, livestock grazing can lead to higher inundation levels and in turn to increased hydrodynamic forces acting on these anthropogenically altered marshes.

Sea level rise and marsh surface elevation change in the Meadowlands of New Jersey

The marshlands of the Meadowlands of New Jersey are valuable wetland ecosystems in a highly developed urban area and provide a natural habitat to more than 285 species of birds, a great variety of fishes, and many other living organisms. It is not clear if these ecosystems and their associated ecological services will persist under conditions of accelerated sea level rise (SLR), in geography where space for a landward retreat of marshlands is limited. In this study, we used the deep rod surface elevation table method and feldspar marker horizons to measure surface elevation change and vertical accretion rate in five marshland sites over 11 years. The controlling parameters of the accretion rate were explored. The results showed that sediments were not limited for vertical accretion. About 16% of the total suspended solids reaching the marsh via the tide was trapped by the marsh surface. Hydraulic duty alone cannot explain differences in deposition rates between low and high marsh. Precipitation, snow accumulation, and sea surge from storms were the main drivers influencing subsidence. The overall subsidence rate was 1.5 ± 1.3 mm/year. All sites combined showed increases in surface elevation of 4.0 ± 0.7 mm/year. This rate of increase is not enough to keep up with the 8 mm/year SLR prediction. There is a 50% chance that in 80 years, 7% of current marshlands will be underwater or will convert to unvegetated mudflats, and most high marsh habitats will disappear.

Life in the aftermath of Marinoan glaciation: The giant stromatolite evolution in the Puga cap carbonate, southern Amazon Craton, Brazil

Giant domal stromatolites associated with vertical tubestone are reported for the first time in the Marinoan (635 Ma) Puga cap dolostone on the southern Amazon Craton, Brazil. The well-preserved giant domal mounds reach 12 m in diameter and 10 m in height, forming laterally continuous biostromes of at least 200 m. The giant mounds occur mainly on the top cap carbonate succession overlying stratiform stromatolites directly underlying glacial diamictites marking the Cryogenian-Ediacaran boundary. We discuss the sedimentological and biological conditions during the greenhouse period that led to the development of these exceptional stromatolitic features. The microbial communities primordially colonized the diamictite substrate, forming stratiform bioconstructions in a shallow platform under hypersaline calm water conditions. The transition from stratiform to domal morphology reflects the progressive deepening of the platform, concomitant with increases in microbial activity. The high production and pre-lithification of carbonate muds, that formed a rigid load-resistant framework, allowed for the accumulation by vertical accretion, increasing the potential for preservation of giant mounds. The progressive generation of accommodation space related to sea-level rises and glacial isostatic adjustment (GIA) caused subsidence in the coastal zone, as proposed by the Snowball Earth hypothesis. The maximum flooding was associated with an increase in siliciclastic inflow, causing burial of microbial communities and the demise of giant stromatolites. The succession was succeeded by the deposition of long-term transgressive lime mud in a CaCO3-oversaturated sea. The giant stromatolites consist of a new element of the Marinoan Puga cap carbonate and provide essential insights to enlarge the discussion about the biological and sedimentological interaction that occurred on post-glacial carbonate platforms worldwide.

Linking testate amoeba assemblages to paleohydrology and ecosystem function in Holocene peat records from the Hudson Bay Lowlands, Ontario, Canada

Peat cores from boreal bog and fen sites in the Hudson Bay Lowlands of Northern Ontario, Canada, were analysed to calculate Holocene carbon accumulation rates, and to show how testate amoeba taxonomic assemblages, inferred depths to water table, and four morpho-traits that may be linked to function (mixotrophy, aperture size, aperture position, and biovolume) changed since peatland initiation. Carbon accumulation rates were on average higher for the Holocene in the fen record (19.4 g C m−2 yr−1) in comparison with the bog record (15.7 g C m−2 yr−1), which underwent a fen-to-bog transition around 6900 cal yr BP. Changes in rates of carbon accumulation were most strongly driven by changes in rates of peat vertical accretion, with more rapid rates in the fen record. Carbon accumulation rates were highest following peatland initiation when reconstructed water tables were highest, and in the late Holocene, when water table positions were variable. Taxa with larger biovolumes and apertures were generally more abundant when reconstructed water tables were higher, most notably following peatland initiation. Mixotrophic taxa were more prevalent in drier conditions and in the bog record. Changing frequencies of morpho-traits suggest that testate amoebae may occupy a higher trophic position in the microbial food web during wetter periods, signaling the possibility of internal feedbacks between peatland ecohydrology and critical ecosystem functions including long-term carbon accumulation.

Beyond 2100: Elevation capital disguises salt marsh vulnerability to sea-level rise in Georgia, USA

Salt marshes rely on sufficient sediment inputs and room for lateral migration to maintain vertical and lateral stability under sea-level rise. As the global rate of sea-level rise accelerates, marshes unable to keep pace become vulnerable to drowning. We evaluated the long-term response of a salt marsh in Georgia, USA, to historical (1935–2018) and future projected rates of sea-level rise. We expected the marsh to be resilient because it receives high sediment inputs and has room to migrate landward. However, sediment cores show marsh accretion (1.55 mm y−1) is lower than the historical rate of sea-level rise (3.25 mm y−1) and that rates are independent of elevation. Results from a vertical accretion model show that while marsh area is stable through 2100 under historical and high sea-level rise scenarios, the marsh relies on elevation capital to maintain its extent under a high rate of sea-level rise. The marsh rapidly loses area beyond 2100 as it depletes its elevation reserve. By 2160, only 12% of the initial marsh area remains. Our results demonstrate that while elevation capital can extend the period of time a marsh maintains its areal extent, it does not remove the long-term threat of drowning when marsh accretion cannot keep pace with sea-level rise.

Combining SfM Photogrammetry and Terrestrial Laser Scanning to Assess Event-Scale Sediment Budgets along a Gravel-Bed Ephemeral Stream

Stream power represents the rate of energy expenditure along a stream reach and can be calculated using topographic data acquired via structure-from-motion (SfM) photogrammetry and terrestrial laser scanning (TLS). This study sought to quantitatively relate morphological adjustments in the Azohía Rambla, a gravel-bed ephemeral stream in southeastern Spain, to stream power (ω), critical power (ωc), and energy gradients (∂ω/∂s), along different reference channel reaches of 200 to 300 m in length. High-resolution digital terrain models (HRDTMs), combined with ortophotographs and point clouds from 2018, 2019, and 2020, and ground-based surveys, were used to estimate the spatial variability of morphological sediment budgets and to assess channel bed mobility during the study period at different spatial scales: reference channel reaches (RCRs), pilot bed survey areas (PBSAs), and representative geomorphic units (RGUs). The optimized complementary role of the SfM technique and terrestrial laser scanning allowed the generation of accurate and reliable HRDTMs, upon which a 1-D hydrodynamic model was calibrated and sediment budgets calculated. The resulting high-resolution maps allowed a spatially explicit analysis of stream power and transport efficiency in relation to volumes of erosion and deposition in the RCR and PBSA. In addition, net incision or downcutting and vertical sedimentary accretion were monitored for each flood event in relation to bedforms and hydraulic variables. Sediment sources and sinks and bed armoring processes showed different trends according to the critical energy and stream power gradient, which were verified from field observations. During flows exceeding bankfull discharges (between 18 and 24 m3 s−1 according to channel reach), significant variations in ∂ω/∂s values and ω/ωc ratios (e.g., −15 < ∂ω/∂s < 15 Wm−3; ω/ωc > 2 for a peak discharge of 31 m3 s−1) were associated with a large amount of bedload mobilized upstream and vertical accretion along the middle reach (average rise height of 0.20 to 0.35 m for the same event). By contrast, more moderate peak flows (≤10 m3 s−1) only produced minor changes resulting in surface washing, selective transport, and local bed scouring.

Salinity and Simulated Herbivory Influence Spartina alterniflora Traits and Defense Strategy

Sea level rise is expected to push saline waters into previously fresher regions of estuaries, and higher salinities may expose oligohaline marshes to invertebrate herbivores typically constrained by salinity. The smooth cordgrass, Spartina alterniflora (syn. Sporobolus alterniflorus), can defend itself against herbivores in polyhaline marshes, however it is not known if S. alterniflora’s defense varies along the mesohaline to oligohaline marsh gradient in estuaries. I found that S. alterniflora from a mesohaline marsh is better defended than plants from an oligohaline marsh, supporting the optimal defense theory. Higher salinity treatments lowered carbon content, C:N, and new stem biomass production, traits associated with a tolerance strategy, suggesting that salinity may mediate the defense response of S. alterniflora. Further, simulated herbivory increased the nitrogen content and decreased C:N of S. alterniflora. This indicates that grazing may increase S. alterniflora susceptibility to future herbivory via improved forage quality. Simulated herbivory also decreased both belowground and new stem biomass production, highlighting a potential pathway in which herbivory can indirectly facilitate marsh loss, as S. alterniflora biomass is critical for vertical accretion and marsh stability under future sea level rise scenarios.

Carbon storage and sediment trapping by Egeria densa Planch., a globally invasive, freshwater macrophyte

Invasive plants have long been recognized for altering ecosystem properties, but their long-term impacts on ecosystem processes remain largely unknown. In this study, we determined the impact of Egeria densa Planch, a globally invasive freshwater macrophyte, on sedimentation processes in a large tidal freshwater region. We measured carbon accumulation (CARs) and inorganic sedimentation rates in submerged aquatic vegetation SAV dominated by E. densa and compared these rates to those of adjacent tidal freshwater marshes. Study sites were chosen along a range of hydrodynamic conditions in the Sacramento-San Joaquin Delta of California, USA, where E. densa has been widespread since 1990. Cores were analyzed for bulk density, % inorganic matter, % organic carbon, 210Pb, and 137Cs. Our results show that E. densa patches constitute sinks for both “blue carbon” and inorganic sediment. Compared to marshes, E. densa patches have greater inorganic sedimentation rates (E. densa: 1103–5989 g m−2 yr−1, marsh: 393–1001 g m−2 yr−1, p < 0.01) and vertical accretion rates (E. densa: 0.4–1.3 cm yr−1, marsh: 0.3–0.5 cm yr−1, p < 0.05), but similar CARs (E. densa: 59–242 g C m−2 yr−1, marsh: 109–169 g C m−2 yr−1, p > 0.05). Sediment stored by E. densa likely reduces the resilience of adjacent marshes by depleting the sediment available for marsh-building. Because of its harmful traits, E. densa is not a suitable candidate for mitigating carbon pollution; however, currently invaded habitats may already contain a meaningful component of regional carbon budgets. Our results strongly suggest that E. densa patches are sinks for carbon and inorganic sediment throughout its global range, raising questions about how invasive SAV is altering biogeochemical cycling and sediment dynamics across freshwater ecosystems.

The Effect of Fertilization on Biomass and Metabolism in North Carolina Salt Marshes: Modulated by Location‐Specific Factors

AbstractThe resilience of salt marshes to sea level rise depends on vertical accretion through belowground biomass production and sediment deposition to maintain elevation above sea level. Increased nitrogen (N) availability from anthropogenic sources may stimulate aboveground biomass production and sediment deposition and, thus, accretion; however, increased N may also negatively impact marsh accretion by decreasing belowground biomass and increasing net CO2 emissions. A study was conducted in Spartina alterniflora‐dominated salt marshes in North Carolina, USA, to determine how responses to fertilization vary across locations with different physical and chemical characteristics. Pore water residence time, inundation time, and proximity to tidal creeks drove spatial differences in pore water sulfide, ammonium, and dissolved carbon concentrations. Although annual respiration and gross primary production were greater at the creek edge than interior marsh sites, net ecosystem CO2 exchange (NEE) was nearly balanced at all the sites. Fertilization decreased belowground biomass in the interior sites but not on the creek edge. Aboveground biomass, respiration, gross primary production, and net CO2 emissions increased in response to fertilization, but responses were diminished in interior marsh locations with high pore water sulfide. Hourly NEE measured by chambers were similar to hourly NEE observed by a nearby eddy covariance tower, but correcting for inundation depth relative to plant height was critical for accurate extrapolation to annual fluxes. The impact of fertilization on biomass and NEE, and thus marsh resilience, varied across marsh locations depending upon location‐specific pore water sulfide concentrations.

Source-to-sink system for peat accumulation in marginal basins of the South China Sea with the Qiongdongnan Basin as an example

In the coal-bearing deposits of the Qiongdongnan Basin in the South China Sea, coal seams are for the most part extremely thin, typically less than 0.5 m. Coal streaks are commonly observed but are not concentrated. In addition, the ash content of the coal components is very high, indicating that the geological conditions of peat accumulation and coal formation were very unstable. For these reasons, it can be speculated that at the time of development, peat mires were relatively poor, in terms of inheritance and continuity, and were commonly not preserved. Coal measures, coal seams (in situ peat accumulation over long distances) and dispersed terrigenous sedimentary organic matter, which together constitute a type of complex coal source-to-sink system found in basin-marginal basins, include three aspects. (1) The material is homologous: whether autochthonous accumulation, allochthonous accumulation or dispersed organic matter is present, the total organic matter source is homologous, i.e. suitable for development and plant growth. (2) The process of organic matter transport (channel) is divided into three forms: vertical accumulation in situ, transport to relatively fixed areas by flowing water, and transport by other mechanisms with long-distance dispersion and drift. (3) There are three types of aggregation forms: vertical accumulation in situ, transport from flowing water to relatively fixed areas away from where plants grow and aggregate, and deposition in deep-water basins fixed with other substances; the latter type requires a period of time for long-distance dispersion and drift, including channel flow and surface flow. KEY POINTS From plant growth to peat formation, the ‘source–channel–sink system’ of coal formation is ‘sensitivity to sedimentary environments and complexity of distribution’. Convergence of coal-series, coal seam (in situ peat aggregation and allochthonous aggregation), and terrestrial dispersed organic matter deposition, constitute a complex coal-measure source–sink system. Source includes substance homology, channels include in situ vertical accumulation and dispersion drift in short and long distances, with convergence forms including in situ vertical accretion, remote sink and dispersion in deep-water basin.

An integrated approach to determine three‐dimensional accretion geometries of tidal point bars: Examples from the Venice Lagoon (Italy)

AbstractLow rates of lateral migration (centimetres to decimetres per year) combined with relatively high rates of vertical accretion (millimetres to centimetres per year) recorded in microtidal channels of the Venice Lagoon (Italy) give rise to point‐bar geometries and internal facies arrangements that differ substantially from widely accepted models of point‐bar sedimentary architecture. In this study, field data from the Venice Lagoon are combined with a three‐dimensional forward stratigraphic model, the ‘Point‐Bar Sedimentary Architecture Numerical Deduction’ (PB‐SAND), to predict the stratal geometries of point bars formed in aggradational settings. The PB‐SAND uses a combined geometric and stochastic modelling approach that can be constrained by field evidence. The model applied determines the geometry of four point bars generated by 9 to 11 m wide channels cutting through salt marshes. An iterative best‐fit modelling approach has been used to obtain multiple simulations for each case study, each of which fits the observations derived from the analysis of time‐series historical aerial photographs and 44 sedimentary cores. Results demonstrate how the geometry of the bars is determined by the development of two key stratal surfaces: the point‐bar brink and channel‐thalweg surfaces. These surfaces are defined by the progressive translation and vertical shift of the point‐bar brink (i.e. break of slope between bar top and bar slope) and the channel thalweg (i.e. deepest part of the channel) during bar evolution. The approach is used to: (i) reconstruct three‐dimensional point‐bar geometries; (ii) propose alternative reconstructions; (iii) provide insight to drive the acquisition of additional data to better constrain the proposed models; and (iv) provide insight into the mechanism of bar growth for slowly migrating channels in settings subject to relatively high rates of aggradation. This study highlights how interaction between styles of planform transformation and latero‐vertical shifts of meandering channels can determine the geometry of related sedimentary bodies.

Elevation and accretion dynamics at historical plots in the Biloxi Marshes, Mississippi Delta

The objectives of this study were to examine changes in accretion and elevation change over periods of up to 15 years for the Biloxi marsh complex (BMC) in southeastern Louisiana, part of the Mississippi Deltaic Plain, identify factors affecting accretionary dynamics, and put these findings in the context of ongoing restoration. We present elevation and accretion data from Surface Elevation Table (SET) and feldspar marker horizon sites first established in 2003. The sites were clustered in two areas (East and West) in the central BMC on the eastern edge of the Mississippi delta. Accretion markers were used in conjunction with elevation measurements to calculate shallow subsidence. These data were analyzed along with similar data from nearby Coastwide Reference Monitoring System (CRMS) sites located around the periphery of the BMC. Elevation decreased at the Western sites by −0.35 ± 0.13 cm/yr, and increased at the Eastern and CRMS sites by 0.40 ± 0.03 cm/yr and 0.72 ± 0.09 cm/yr, respectively. The rate of accretion was similar at the Western (0.49 ± 0.14 cm/yr) and Eastern (0.64 ± 0.07 cm/yr) sites, and over twice as much (1.30 ± 0.11 cm/yr) at the CRMS sites. Shallow subsidence, calculated as the difference between vertical accretion and surface elevation change, was 0.76 ± 0.49 cm/yr at the Western sites, 0.23 ± 0.06 cm/yr at the Eastern sites, and 0.58 ± 0.11 cm/yr at the CRMS sites. These trends are consistent with the observation that sediment is brought in from Chandeleur Sound to the east and is attenuated as deposition occurs across the landscape from east to west, and that levee flank depressions associated with Bayou La Loutre, an abandoned Mississippi River distributary ridge, are causing locally high subsidence in the Western region. Without intervention, these localized areas of the Western region will be submerged within the next several decades at current rates of elevation loss and eustatic sea-level rise, while the Eastern sites and the wetlands on the periphery of the BMC are likely to keep pace with sea level rise well into the second half of this century. These results demonstrate the importance of accurate knowledge of both subsidence and accretionary dynamics in determining coastal wetland sustainability and restoration approaches.

Suspended particulate matter for sediment accumulation on inundated anthropogenic marshland in the southern North Sea – Potential, thresholds and limitations

Determining the availability of suspension load, i.e. suspended particulate matter (SPM), in shallow marine tidal systems is necessary to estimate whether coastal lowlands may experience vertical accretion rates that keep pace with global sea level rise. In particular anthropogenic marshland areas are affected by limitations of SPM accumulation due to artificial structures such as dikes and revetments that act as transport barriers.With the use of an autonomously operating turbidity measuring network installed at and around the Hallig Langeness in the North Frisian Wadden Sea, we have analyzed eight individual inundations on this island-like marshland during harsh weather conditions. The results reveal that the majority of the available SPM could not accumulate on the marshland due to adjacent coastal protection measures. This reduction in SPM affects especially grain sizes within the very fine to fine sand range between 63 and 200 μm that can account for up to 70% of the sampled SPM during an inundation. Furthermore, we have determined a tidal average SPM concentration threshold of 0.15 g/l should be available on the tidal flat in order to allow sufficient vertical marshland accretion. Although this threshold is exceeded by the determined SPM on the tidal flats around Langeness, the available SPM on the marshland after passing the coastal protection measures is at most 20%. Hence, an optimized coastal management strategy has the potential to increase the sediment accumulation rate on the marshlands by a factor of five, which might allow for keeping pace with local sea level rise.

Re-evaluating mid-Holocene reef “turn-off” on the inshore Southern Great Barrier Reef

In the face of changing global climate the future of corals reefs is uncertain. High latitude reefs may offer potential refugia for corals under projected increasing sea surface temperatures (SSTs). To understand the reef growth potential of modern high latitude reefs it is first necessary to understand past reef growth and response to climatic and environmental changes. The history of the Great Barrier Reef (GBR) has been shown to be punctuated by a multi-millennial Mid-Holocene reef “turn-off” or initiation hiatus in the Northern and Southern GBR (∼5500–2500 years before present [yr. BP]). Here we present the results of chronologically constrained reef matrix cores from five continental island fringing reefs and coral communities in the Keppel Islands to revaluate the timing, extent and possible drivers of the Mid-Holocene hiatus in the Southern GBR. Earliest initiation occurred at Wedge and Halfway Islands at 7773 ± 19 yr. BP and 7455 ± 20 yr. BP, respectively. Following initiation, vertical reef accretion at Halfway was comparable to previously reported rates for the GBR of ∼3.4 mm yr−1, increasing to ∼8.0 mm yr−1 between 6100 and 5500 yr. BP. Conversely, the coral community at Wedge Island, located closer to mainland terrestrial influence, accreted <1.0 mm yr−1 to 5500 yr. BP after which a 3000-year stratigraphic hiatus is observed. A negative relative sea level oscillation at 5500 yr. BP coincides with the transition of Halfway reef from vertical accretion to lateral progradation and the “turn-off” of the Wedge Island coral community. The reef at Middle Island appears to have initiated in the midst of the Mid-Holocene hiatus period at ∼3500 yr. BP, with vertical accretion at both Middle and Halfway Islands being the highest for the region’s history at 12.5 ± 3.3 mm yr−1 and 15.0 ± 6.0 mm yr−1, respectively, between 3500 and 3000 yr. BP. A proposed late-Holocene relative sea level highstand of ∼1 m at ∼2000 yr. BP coincides with the first dates identified at near-shore Divided Island, and the re-initiation of coral growth at Wedge Island, although both showed limited net vertical accretion. These near-shore sites “turned-off” again between 1300 and 900 yr. BP, likely associated with a rapid relative sea level fall. Reef progradation also slowed at Halfway Island and Middle Island after 1200 yr. BP until a recent increase in reef growth at Middle Island since the late 1970’s associated with modern increases in average SSTs. Our data suggests that although reef-scale “turn-off” events occurred, there is no evidence for a regional scale hiatus in coral growth or initiation comparable to that found further north on the GBR. A comparison with other Southern and south-Central GBR reef cores suggests that relative sea level oscillations in conjunction with changes to climate, appear to have driven varying, but synchronous, modifications to reefs at 5500, 4600, 2800 and 1200 yr. BP.